Metallic contact flip-chip interconnect bonding has been successfully used for many years to provide mechanical and/or electrical connection between two or more electrical, optical, and-or mechanical components. An example of the flip-chip interconnect would be the face-to-face indium bump bonding of an optical detector chip to its corresponding readout integrated circuit (ROIC) chip to form an infrared detector focal plane assembly. Many other types of 3-dimensional hybrid assemblies utilize similar metallic connection schemes. It is understood in the industry that removal of native oxidation from the surface of the metallic contacts prior to bonding results in a much improved electrical and/or mechanical interconnection by removing the relatively thick and tough indium oxide layer from the contacts, thus allowing metal-to-metal bonding without the impediment of the tough oxide layer at the interface. This elimination of interfacial oxide also improves ohmic contact between the two surfaces by eliminating the non-conducting or semiconducting metal-oxide interfacial layer.
Various prior art methods of oxide removal have been devised including acid chemical etching as taught by Schulte and Olson in U.S. Pat. No. 4,865,245 to improve hybridization cold-welds, and vacuum plasma etching of the metallic contacts prior to hybrid bonding. Each of these methods has specific drawbacks. Example drawbacks of acid chemical etching include the potential for liquid-borne or airborne contamination of the devices, handling damage to delicate chips, a residue of acid etchant on the surface of the components which can lead to reliability problems, cost of chemicals and their subsequent disposal, a slow process turnaround time, usage of toxic and dangerous chemicals, the need for corrosion-resistant etching hardware and enclosures, high maintenance requirements, the need for toxic/corrosive exhaust provisions, and unwanted chemical reactions between the etchant and other surface features of the chips. Example drawbacks of vacuum plasma etching for oxide removal include expensive vacuum plasma equipment, slow process turnaround time, potential for plasma damage to the components by hot electrons, ions, and high kinetic energy atoms, back-sputtering of unwanted metals from vacuum chamber components or from the substrate itself onto the substrate being cleaned, expensive and time-consuming maintenance requirements of the required equipment, and a higher level of operator proficiency and training needed to run sophisticated vacuum plasma systems.
Additionally, and very significantly, these methods only very temporarily remove oxidation from the metallic contacts, since the oxide regrows rapidly when exposed to air after the oxide reduction process. If the bonding cannot be performed in a very short period of time, and/or if the bonding is performed at elevated temperature, the regrown oxide inhibits bonding of the metallic contacts. The thicker the layer of regrown oxidation, the more compression and deformation of the contacts are required to obtain even marginal metal-to-metal cold welding.
Prior art also teaches deposition of an oxidation-inhibiting layer on the deoxidized surface, but that layer must be removed prior to bonding. This poses two problems: 1) equipment and process time required to remove the oxidation inhibiting layer are costly and time-consuming; and 2) once the oxidation inhibiting layer is removed, the surface is once again subject to re-oxidation prior to the bonding. To offset these drawbacks, the removal of the passivating layer would normally involve the use of a vacuum chamber or confinement chamber to facilitate the chemical reaction and also to slow the re-oxidation process. These chambers impose additional time, expense, bulk, and complexity to the bonding process and equipment.
What is needed is a rapid, non-damaging, inexpensive, simple process for metallic oxide removal which also produces a modified surface that inhibits the regrowth of oxide while at the same time not hindering bonding ability, so that it need not be removed prior to bonding; thus forming electrically and/or mechanically robust interconnect bonds.